Former National Security Agency analyst Edward Snowden would not face the death penalty for leaking secret information on surveillance programmes, the Attorney General has said.

In a letter dated July 23, Eric Holder said the criminal charges the ex-NSA worker faces do not carry the death penalty and that the US would not seek it even if Snowden is charged with additional crimes that were eligible for it.

Mr Holder said the letter follows reports that Snowden has filed papers seeking temporary asylum in Russia on the grounds that if he were returned to the US he would be tortured and face capital punishment.

The letter was sent to Alexander Vladimirovich Konovalov, Russian minister of justice.

In it, Mr Holder said: "I can report that the US is prepared to provide to the Russian government the following assurances regarding the treatment Mr Snowden would face upon return to the United States.

"First, the US would not seek the death penalty for Mr Snowden should he return to the US. Mr Snowden will not be tortured. Torture is unlawful in the US."

The letter added that he would be brought before a civilian court and would receive "all the protections that US law provides".

Whistleblower Snowden, who leaked information on largely secret electronic surveillance programmes, remains in a transit area at Moscow's Sheremetyevo airport after travelling there from Hong Kong last month.

His passport has been revoked, but Mr Holder said he could still travel out of Russia because he remains a US citizen and is eligible for a limited validity passport which would allow him to fly directly to the US.

Russian President Vladimir Putin has said Snowden can only be granted sanctuary in Russia if he stops actions that harm the US.

He has been offered asylum by three Latin American countries, but none of them is reachable by a direct flight from Moscow.

Snowden's father, Lon Snowden, said in an interview on NBC's Today show that some members of US Congress were trying to "demonise" his son.

He said politicians should be more focused on whether the NSA's collection of the phone records of millions of Americans is constitutional.

The House voted 217-205 on Wednesday to keep the NSA surveillance programme running.